Magnetic coating of iron surface

ABSTRACT

A METHOD OF INIHIBITING CORROSION OF AND IMPARTING CORROSION RESISTANCE TO A SURFACE OF AN IRON ARTICLE BY FORMING A MAGNETIZED COATING UPON THE SURFACE THEREOF EITHER THROUGH EXPOSURE OF THAT SURFACE TO A MAGNETIZATION TREATMENT DIRECTLY WITHOUT ANY PRE-TREATMENT FOR REMOVAL OF THE OXIDE FILM FROM THE SURFACE THEREOF OF SUCH EXPOSURE AFTER PICKLING OF THE SURFACE. THE MAGNETIZATION TREATMENT CAN INVOLVE EXPOSURE TO A MAGNETIC FIELD, OR THE APPLICATION OF A COATING OF A MAGNETIZED MATERIAL INCLUDING A MATERIAL HAVING STRONG MAGNETIC SUSCEPTIBILITY. THE MAGNETIZED MATERIAL CAN BE APPLIED TO THE SURFACE AS A COATING PRIOR TO PICKLING OR SUBSEQUENT TO PICKLING OF THE SURFACE AND THE COATING OF THE MIXTURE CAN CONSIST OF ONE OR SEVERAL MAGNETIZED HIGHLY MAGNETICALLY SUSCEPTIBLE MATERIALS, OR A MIXTURE OF SUCH MATERIALS AND AT LEAST ONE WEAKLY OR NONMAGNETICALLY SUSCEPTIBLE MATERIAL; AND, ALSO MAY CONSIST OF A MAGNETIZED MATERIAL ALONE OR MIXED WITH A WEAKLY OR NON-MAGNETICALLY SUSCEPTIBLE MATERIAL, WHEN ALSO MIXED IN A CARRIER VEHICLE.

United States Patent Oflice Patented Jan. 9, 1973 3,709,730 MAGNETIC COATING OF IRON SURFACE Kenkichi Tsukamoto, 462-1 Oaza-Taki, Fujiwara-machi, Shioya-gun, Toclligi-ken, Japan No Drawing. Filed Oct. 14, 1968, Ser. No. 767,521 Claims priority, application Japan, Oct. 21, 1967, 42/67,618 Int. Cl. B44d 1/02 US. Cl. 117-238 Claims ABSTRACT OF THE DISCLOSURE A method of inhibiting corrosion of and imparting corrosion resistance to a surface of an iron article by forming a magnetized coating upon the surface thereof either through exposure of that surface to a magnetization treatment directly without any pre-treatment for removal of the oxide film from the surface thereof or such exposure after pickling of the surface. The magnetization treatment can involve exposure to a magnetic field, or the application of a coating of a magnetized material including a material having strong magnetic susceptibility. The magnetized material can be applied to the surface as a coating prior to pickling or subsequent to pickling of the surface and the coating mixture can consist of one or several magnetized highly magnetically susceptible materials, or a mixture of such materials and at least one weakly or nonmagnetically susceptible material; and, also may consist of a magnetized material alone or mixed with a weakly or non-magnetically susceptible material, when also mixed in a carrier vehicle.

This invention relates to magnetic coating of iron surface, and more specifically to a process for magnetic coating of iron surface which comprises subjecting an iron article directly to a magnetization treatment without any pretreatment for removal of the oxide film from the surface, or to a magnetic field treatment through even and close contact wtih an already excited ferromagnetic material, or to a magnetic field treatment through application of an excited ferromagnetic material after the descaling (pickling) of the iron surface.

Originally iron articles easily corrode with rust and are usually coated with a corrosion-resistant paint when in use. Particularly good protection against rust must be provided where they are to be exposed to high humidity or briny air.

Rustproofing technique most commonly in practice heretofore has been to mix an active pigment such as red lead or metallic zinc powder with a suitable vehicle and apply the mixture as an anticorrosive coating material to the iron surface.

According to this conventional method, it has been difiicult to maintain a durable rustproof elfect in a rigorous atmosphere because such active pigments are highly instable chemically and are easily worn out or are deteriorated. Among other technical problems yet to be solved of the conventional practice are the limitations to the applications of such coating materials, for example for specific effect of waterproofness or acidproofness by reason of the particular vehicle properties.

Especially, the derusting which is resorted to as a preparatory process to prevent subsequent peeling of the coated material together with rust and thereby to extend the life of the coated film, requires much time and cost.

It is therefore an object of the present invention to provide a process for magnetic coating of iron surface whereby a coated film which is durable and resistant to physical impacts, and has particularly good resistance to saline water can be formed on iron surface.

Another object is to provide a process for magnetic coating of iron surface whereby the rust on iron surface is converted into magnetite (Fe O which is more resistant to corrosive attacks than ordinary iron and hence any careful derusting (as by blasting) of the surface which is otherwise needed can be omitted.

The surface of iron article such as rolled steel plate that has not been pickled is covered with a film of iron oxide (scale). This oxide film is formed by the outward outward diffusion of Fe and inward diffusion of oxygen, and usually consists of three layers, i.e. of hematite (-Fe O magnetite (Fe O and bistite (FeO), the lastmentioned layer being lowermost.

Magnetite that constitutes the intermediate layer is a substance capable of becoming a ferromagnetic permanent magnet and yet has a higher potential than iron and possesses highly stabilized corrosion resistance.

This invention concerns a process for controlling the corrosive reaction due to the intrusion of water and oxygen through cracks and pinholes by cooling rolled steel plates, steel pipes and other iron articles during their manufacture to a temperature below the demagnetization temperature (Curie point), thereby subjecting the articles to a magnetization treatment in order to excite the intermediate layer of magnetite and magnetically increase the adhesion between that layer and the underlying surface of the articles.

The invention is also directed to provide articles which are subjected, where necessary, to a magnetic coating treatment simultaneously with the magnetization treatment, thereby conferring a great corrosion resistance on the articles.

The pigments for the coating materials to be employed in this case are, from the viewpoint of the object of the invention, preferably gamma-hematite ('yFe O chromite, chromium oxides (CrO and Cr O magnetite, superfine iron dust, manganese-aluminum, manganese-bismuth (MnB), barium ferrite (BaO.6Fe O and various other ferrites. Chromium oxide (CrO is particularly preferred.

The present invention also provides a treatment process for [forming on the surface of iron articles a coated film having good durability, resistance to physical impacts and particularly good resistance to saline water by applying thereon a magnetized coating material as the pigment, prepared by magnetizing the mixed powders of more than one kind of strongly magnetically susceptible substance such as gamma-hematite, magnetite, superfine iron powder, spinel type crystal structure ferrite, magnetic plumbite type crystal structure ferrite, garnet type crystal strucure ferrite, perovskite type crystal structure ferrite, and of more than one kind of weakly or non-magnetically susceptible substance such as chromium oxide, chromite, manganese dioxide, alumina, silica, for example, in a DC field of about 12,500 gauss for more than second. Besides, as is generally known, magnetization does not restrict magnetizing field intensity and it is enough for magnetic field intensity where ferromagnetic substance content saturates magnetically.

The present invention concerns a treatment process for providing iron articles having an outer coated film of the above properties which comprises the steps of applying the powder of a substance capable of being magnetized on the above articles or deoxidized and descaled (pickled) articles, drying the coated surface, subjecting the dried surface to a magnetic field treatment and magnetizing said surface.

According to the present invention, it is possible to fill a coating material comprising as the pigment the powder of a magnetized substance in an applicator, e.g., a sprayer provided with a powerful permanent magnetic field at magnetized at the time of issuance from the nozzle orifice.

It is further possible to provide a similar permanent magnetic field in the coating brush so as to magnetize the contained pigment at the time of application.

In the practice of the present invention, the vehicle to be mixed with the magnetized powder is a cement mortar or high polymer selected from the class consisting of alkyd, vinyl, tar, rubber chloride, epoxy and other synthetic resins and natural oils and fats.

Further, in the present invention, a DC magnetic field for magnetization from the outside is not always essential but spontaneous magnetization can be effected by finely pulverizing ferromagnetic chromium oxide, manganesealuminum, manganese-bismuth, barium ferrite or the like which is chosen as a material to be subjected to the magnetization treatment to a particle size within the critical range of diameter for the respective unit magnetic domain that is known in the art.

Provided, however, that it is important according to this invention that chromium oxide (CrO or chromite must be incorporated where the magnetized body of gamma-hematite or magnetite is employed as the pigment.

Critical diameters of unit magnetic domains of ferro- *Oritical diameter where there is a magnetic domain wall but which ails to function as such.

Regarding the standard amount of coating in accordance with the present invention, it will be appreciated that the amount depends on the condition of iron surface; a corroded surface, for example, is porous and uneven, and the real area to be coated is governed by the degree of surface irregularity. Usually the unevenness of rusted surface represents a thickness of about 0.075 mm., and therefore the coated film in this case is required to have a thickness of about 0.038 mm. or more. Thus, it is preferable to apply the coating material in an amount equivalent to about 0.3 kg. per sq. m. Upon a treatment in accordance with the present invention, the coated material reacts with the rust (Fe O .XH O) which is present on the iron surface and is converted into a rustproof and corrosion-resistant magnetite (Fe O This conversion occurs not only in the rust present on the iron or steel surface before coating but equally in the rust which is secondarily produced under the coated film after coating. Thus, the coated iron article attains a high degree of corrosion resistance because it is shielded and protected from water, air and other corrosive factors by virtue of not merely the coated film but also of the layer of magnetite formed by the conversion of rust.

This magnetite layer is stable both physically and chemically and its lattice constant is equal to that of the iron surface. Hence it solidly combines with the iron surface. Also, because magnetic attraction works among the particles of magnetic pigment and between the iron surface and the pigment particles, a tridimensionally dense and bidimensionally orderly network structure is formed. This confers a strong resistance to physical impacts on the coated film and renders it rustproof for an extended period of time.

The above reaction can be clearly pursued by an X-ray diffraction (analysis).

Ordinary coating materials are such that the deteriore en 9 h 99%??8 fi m? ye-s e immed y f e drying and proceeds with time until the coated films swell, contract, crack or otherwise become deteriorated with a corresponding loss of the rustproof effect. According to the treatment process of the invention, the coated film is rather hardened with increased strength.

The present invention is now illustrated by the following examples.

EXAMPLE 1 A test specimen was prepared by mixing 25% of a mixed powder consisting of magnetized y-Fe o having magnetic characteristics of Bm=500 gauss/s Br=235 gaus/s Hc=76 (oe.) Br/Bm=0.43

and 20% CrO 9% of two types of red oxide, 8% of zinc flower, 21% of white zinc, 25 of boiled oil, 7% of long oil varnish, 0.5% of antifiowing agent (aluminum stearate), 2% of a dryer solution, and 2.5% of mineral spirit. This test specimen and commercially available red lead were applied under the same conditions on an ordinary iron plate (to form a coated film in an amount equivalent to 0.42 g./cm. and were subjected to a salt water spraying test (in conformity to the HS testing procedure). The former would not corrode with no change at all after hours, whereas the latter began rusting in only 40 hours.

EXAMPLE 2 Test pieces used were a rolled steel sheet A measuring 100 mm. x 200 mm. x 2.5 mm. and which had a surface layer of oxide scale (magnetized in a DC magnetic field of 300T x 600A at 26 C. for one minute and coated with the magnetic coating material of Example 1 in an amount equivalent to 5 g./drn. and a sheet B (same as A but not magnetized, coated with 5 g./dm. of the red lead anticorrosive paint of Example 1). These test pieces were kept immersed in saline water and their weight losses were measured. The comparative values were as given in Table 2 (the comparative values being totals for the year).

The anticorrosive rate attained is defined by the following formula:

Anticorrosive rate Amount of corrosion Amount of corrosion of nonmagnetized -of magnetized steel steel sheet sheet Amount of corrosion of nonmagnetized steel sheet TABLE 2 Testing period Sept. 1965-1966. No. of days 368. Corrosion rate of tested steel sheet (g/d m? Y):

Nonmagnetized 69.30.

Magnetized 8.35. Anticorrosive rate, percent 88.

While the composition of the oxide scale on iron surface is somewhat dependent upon the temperature employed for the heat treatment, the major constitutent of the composition is Fe whose magnetic characteristics are:

Hc=18-120 (oe.) Im=58-450 (gauss) For the magnetization purpose, a DC magnetic field of an ampere turn of more than times of Hc Lg (where He is the coercive force and Lg is the length of gap parallel to the magnetic flux) is added.

Bistite that has been referred to above is decomposed at a temperature below 575 C. into magnetite and metallic iron.

Production of hematite can be reduced by adjusting the concentration of oxygen during the heat treatment.

In the mechanism of chemical reaction in accordance with the treatment process of the invention, factors of the greatest interest and benefit are the following three, though they have not been fully clarified yet. First, in the present process, the strong adhesion of the coated film to the iron surface combines with the catalytic action of magnetized Fe Od, to induce an aqueous replacement of the hydrate of hematite which has become porous and acquired an water-absorptive property. Also, the molecular chains produced on the magnetic poles, unlike the side chains independent of the magnetic poles, restrict free revolving motions and bring an increased number of hydrogen bonds. As a whole, therefore, the effect of a magnetic field ensures better orientation and orderly arrangement of the molecular groups of iron oxide or the individual iron oxide molecules in the magnetizing direction. Thus, any disorderly arrangement for the conversion into magnetite (Fe O is modified to re-orient the molecules themselves to an adequate pattern. Second, an extremely satisfactory rustproof effect can be achieved through reduction of the amount of corrosion by changing the velocity of corrosion changed particles in accordance with the following relative formula based on known principles:

where M: mass of electrically charged particle 2: charge on charged particle V: velocity of charged particle R: radius of motion of charged particle H: intensity of magnetic field (gauss) Third, the magnetic coated film formed over the iron surface effectively keeps water or oxygen or both away from the iron surface by virtue of the increased adhesion due to the magnetic attraction and inhibits any localized cathode reaction 4H O+20 +8e- 80H) thereby displaying most satisfactory results.

What is claimed is:

1. A method of inhibiting corrosion of and imparting corrosion resistance to a surface of an iron article by forming a magnetized coating upon the said iron surface.

2. A method according to claim 1 in which the magnetized coating is applied by subjecting the iron surface to a magnetic treatment directly without any pre-treatment removal of any oxide film which is present thereon.

3. The method according to claim 1 in which the mag netized coating is applied by substantially evenly coating the iron surface with powders containing magnetized strongly magnetically susceptible materials.

4. The method according to claim 3 in which the mag netized coating is applied subsequent to pickling of the iron surface.

5. The method according to claim 1 in which the magnetized coating is formed upon the iron surface subsequent to the removal of any oxide film present thereon.

6. The method according to claim 1 in which the magnetized coating is applied by coating the iron surface With a mixture consisting of a premagnetized highly magnetically susceptible material and a Weakly or non-magnetically susceptible material.

7. The method according to claim 3 in which the magnetized coating consists of a magnetized highly magnetically susceptible material selected from one or more of the group consisting of gamma-hematite, magnetite, superfine iron powder, spinel type crystal structure ferrite, magnet plumbite type crystal structure ferrite, garnet type crystal structure ferrite and perovskite type crystal structure ferrite.

8. The method according to claim 1 in which the magnetized coating is formed by the steps of, magnetizing a mixture of strongly magnetically susceptible material and weakly or non-magnetically susceptible materials, wherein the strongly magnetically susceptible materials are selected from the group consisting of gamma-hematite, magnetite, superfine iron powder, spinel type crystal structure ferrite, magnet plumbite type crystal structure ferrite, garnet type crystal structure ferrite and perovskite type crystal structure ferrite, and the weakly or non-magnetically susceptible materials are selected from the group consisting of chromium oxide, chromite, manganese dioxide, alumina, and silica; mixing the magnetized mixture with a carrier vehicle and then bringing the thus magnetized mixture into close contact with the iron surface thereby magnetically treating said surface.

9. The method as claimed in claim 8 in which the mixture is exposed to a DC. magnetic field of about 12,500 gauss for less than three minutes.

10. A method as claimed in claim 8 wherein the vehicle which is mixed with the magnetized substance is selected from a group consisting of a cement mortar and a high polymer which is selected from the group comprising alkyd, vinyl, tar, rubber chloride, epoxy and other synthetic resins and natural oils and fats.

11. The method according to claim 8 in which the step of applying the magnetized mixture to the iron surface is performed subsequent to pickling of said surface.

12. The method according to claim 1 in which the magnetized coating is formed by subjecting the surface to a magnetization treatment by applying thereto a strongly magnetized material which contains one or more of strongly magnetically susceptible substances selected from the group consisting of gamma-hematite, magnetite, superfine ll'OIl powder, spinel type crystal structure ferrite, magnetic plumbite type crystal structure ferrite, garnet type crystal structure ferrite and perovskite type crystal structure ferrite, said substances being capable of being magnetized by exposure to a DC. magnetic field of an ampere turn of more than ten times that of the Hc Lg where He is the corrosive force and Lg is the length of gap parallel to the magnetic flux and exposing the coating surface to said D.C. magnetic field.

13. The method according to claim 12 in which said strongly magnetically susceptible substance is mixed with at least one of a weakly or non-magnetically susceptible substance having the critical diameter range for the respective unit magnetic domain under 1.5.

14. The method according to claim 1 wherein the magnetized coating is applied by treating the iron surface with a magnetized substance comprising at least one of the group consisting of gamma-hematite, magnetite, superfiue iron powder, spinel type crystal structure ferrite, magnet plumbite type crystal structure ferrite, garnet type crystal structure ferrite and perovskite type crystal structure ferrite and said substance is reduced to a particle size under 1.5 within the critical diameter range for the respective unit domain and one of chromium surface.

References Cited UNITED STATES PATENTS Swan 148-635 Given 148-635 Vermit 148-635 Rogers 117-236 X Wilhelm et a1. 117-130 X Matsumoto et a1. 252-6551 X 8 Bisschops et a1. 252-6251 X Elser et a1. 148-6 Osterheld 220-64 Kebrich 106-14 X Robinson 148-6 Lunt et a1. 336-221 X Flowers 117-235 Okuda 336-110 X 10 EDWARD G. WHITBY, Primary Examiner US. Cl. X.R.

21-2.5; 106-14; 117-932, R 133, 135, 135.1, R, 160 ZB; 252-6253, 62.54, 62.55, 62.58, 62.59 

